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I'm reading "Patterns, Principles, and Practices of Domain-Driven Design". The book suggests that properties of an Entity should be value objects in order to model domain's ubiquities language. I've seen many examples like EmailAddress or Age with only one field to model domain concepts. I'm confused about it. Is every property of an Entity a value object? Can you provide examples when we can use normal languages provided data types for properties?
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Value Object is an object that represents a concept from your problem Domain. It is important in DDD that Value Objects support and enrich Ubiquitous Language of your Domain. They are not just primitives that represent some values - they are domain citizens that model behaviour of your application.
The answer to the question of "Can we consider an entity as a value object that compares only its Id for equality?" boils down to three parts: In terms of identity comparison, the answer is yes. In terms of immutability, the answer is yes.
The main difference between entities and value objects lies in the way we compare their instances to each other. The concept of identifier equality refers to entities, whereas the concept of structural equality - to value objects. In other words, entities possess inherent identity while value objects don't.
In domain-driven design, an entity is a representation of an object in the domain. It is defined by its identity, rather than its attributes. It encapsulates the state of that object through its attributes, including the aggregation of other entities, and it defines any operations that might be performed on the entity.
In his book, Domain Driven Design (DDD), Eric Evans encourages the use of Value Objects in domain models – immutable types that are used as properties of entities. You can learn more about value objects and DDD in the Domain-Driven Design Fundamentals course which I co-authored with Steve Smith.
1 Identifier vs structural equality: Entities have identifier,entities are the same if they have the same identifier. ... 2 Mutability vs immutability: Value Objects are immutable data structures whereas entities change during their life time. 3 Lifespan: Value Objects Should belong to Entities
Domain-Driven Design (DDD) consists of several building blocks. Two of the most important ones are Entities and Value Objects (VOs). In my last post about Domain-Driven Design (DDD) I explained why DDD is (still) relevant and what it actually is.
An object that represents a descriptive aspect of the domain with no conceptual identity is called a Value Object. Value Objects are instantiated to represent elements of the design that we care about only for what they are, not who or which they are. Think back to the student example in the Entities section.
No, not every property of an entity is a value object.
Properties of entities are one of the following:
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I think that your real question is: Is every value object a Class? Because you can think that for the Age a Java Integer can be enough and this is true. So you have in your entity Person a value object Age of type Integer, there is no need of an age type.
OOP also says that an object is state + behaviour. In your Age case, I assume that it has no behavior so a simple primitive or wrapper class will do the trick, in fact I would go with option this because is simpler.
My advise is, go with a primitive/wrapper class and if you advert that some behavior is needed in that value object, make a class/type.
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